Fibrous architectures are among the most abundant load-carrying materials in nature. This structural motif bridges the length scales from the smallest peptide folding motifs to protein materials as in collagen up to larger structural entities like spider silk or cellulose microfibrils (P. Fratzl, R. Weinkamer, Prog. Mater. Sci. 2007, 52, 1263; M. A. Meyers, P. Y. Chen, A. Y. M. Lin, Y. Seki, Prog. Mater. Sci. 2008, 53, 1) The latter are composed of highly aligned native crystalline β-D-(1-4)glucopyranose polysaccharide chains (cellulose I crystals) where the chains are strongly intermolecularly bound via a multitude of hydrogen bonds. These microfibrils are the main building blocks of plants and are responsible for the mechanical strength of wood or nutshells. Their underlying native nanocellulose fibrils (also known as nanofibrillated cellulose (NFC) or microfibrillated cellulose (MFC)) can be isolated via consecutive chemical/enzymatic and homogenization treatments (I. Siro, D. Plackett, Cellulose 2010, 17, 459; S. J. Eichhorn, A. Dufresne, M. Aranguren, N. E. Marcovich, J. R. Capadona, S. J. Rowan, C. Weder, W. Thielemans, M. Roman, S. Renneckar, W. Gindl, S. Veigel, J. Keckes, H. Yano, K. Abe, M. Nogi, A. N. Nakagaito, A. Mangalam, J. Simonsen, A. S. Benight, A. Bismarck, L. A. Berglund, T. Peijs, J. Mater. Sci. 2010, 45, 1.) They typically consist of highly crystalline nanoscale fibrils with diameters around 1-35 nm and several micrometers in length. Cellulose nanocrystals, alternatively called as nanowhiskers, are related materials, which are shorter and rod-like due to strong acid hydrolysis. Nanofibrillated cellulose is a remarkable emerging class of nature-derived nanomaterial for its extraordinary mechanical properties, combining astonishing stiffness and expected strength with a lightweight character. It has been shown earlier that cellulose I crystals can reach a Young's modulus of up to 136 GPa and an expected strength in the range of a few GPa (S. Iwamoto, W. H. Kai, A. Isogai, T. Iwata, Biomacromolecules 2009, 10, 2571; H. Yano, J. Sugiyama, A. N. Nakagaito, M. Nogi, T. Matsuura, M. Hikita, K. Handa, Adv. Mater. 2005, 17, 153) These properties rank them at the top end of high-performance natural materials. As a comparison, the stiffness of cellulose I is two to three times higher than that for glass fibers (50-80 GPa), just above typical titanium alloys (105-120 GPa) and it approaches that of steel (200 GPa). Strikingly, all of this is realized by a purely organic material with a comparably low density (ca. 1.6 g/mL). This renders cellulose nanofibrils one of the most promising building blocks for future materials.
Furthermore, NFC is based on a natural polymer that is abundant in nature and is renewable and degradable. Therefore, nanofibrillar cellulose might be an interesting constituent in structures where strength is needed.
Individual cellulose polymers have a long history in the context of fiber production. Fibers based on dissolved and regenerated or fully hydrolyzed cellulose and its derivatives (e.g Rayon™) are widely used for textiles or reinforcements, owing to decades of development. However, due to their inherent strength, as originating from the crystalline character, as not preserved in dissolution processes, NFC based materials possess the potential to go significantly beyond the mechanical performance of molecular cellulose materials.
Thus, numerous trials have been made on trying to achieve nanocomposites based on NFC and synthetic engineering plastics. The reported experiments have shown properties lower than desired, especially with hydrophobic thermoplastics, which would be the most important matrix polymers. The main reason for that is the difficult nature of the NFC: water is needed to fully disperse pristine NFC in the nanoscale. If the water is removed during the compounding stage, the fibers aggregate and phase separation takes place, which will lead to poor mechanical properties.
NFC production techniques are based on grinding (or homogenization) of aqueous dispersion of pulp fibers possibly combined with chemical/biochemical treatments. The concentration of NFC in dispersions is typically very low, usually around 1-5%. After the grinding process, the obtained NFC material is a dilute viscoelastic hydrogel. At very small concentrations, the NFC material in water forms a viscous fluid.
Thus, there is an evident need for transforming the aqueous NFC raw material to a structure where the water is essentially absent and the nanocellulose fibrils are arranged so that they can be used as structural parts in composites or as fiber-like structures of high strength.
In order that the NFC can be used as various structural constituents, water must be removed from the NFC hydrogel. The fundamental problem in mechanical water removal is the high hygroscopicity of NFC and the ability of NFC hydrogel to form a very dense and impermeable nanoscale membrane around itself, for example during filtration. The formed membrane hinders the diffusion of water from the gel structure, which leads to very slow water removal rates. However, water removal is not the only problem, but the nanocellulose fibrils must be arranged in a structure where their strength potential can be fully utilized. Whenever water is removed, the nanocellulose fibrils tend to aggregate which results in poor mechanical properties of the product.
The article Capadona J. R. et al. A versatile approach for the processing of polymer nanocomposites with self-assembled nanofiber templates, Nature Nanotech. 2, 765-769 (2007) describes gels made of nano-scale cellulose whiskers which are obtained through acid hydrolysis of tunicate mantles. The whiskers exist initially in aqueous dispersion and they are made to an organogel in a sol-gel process through extraction agent exchange with a water-miscible extraction agent, whereafter the gel is filled with a matrix polymer by immersing the gel in a solution of the polymer and dried. During the gel-forming step acetone was introduced on top of the aqueous whisker dispersion without mixing the layers. The acetone was exchanged daily and the acetone layer was gently agitated to promote the extraction agent exchange. After some days the acetone organogel, called a “scaffold” was obtained. The article also reports the use of acetonitrile, ethanol, methanol, isopropanol and tetrahydrofuran as extraction agents for making the organogel. The gelled nanofiber scaffold was impregnated with a polymer by immersion in a polymer solution, and the nanocomposite was dried and compacted. Using this approach, nanocomposites with polybutadiene and polystyrene could be fabricated with improved mechanical properties. However, the gel forming step through the extraction agent exchange takes typically many days. No essentially pure NFC fiber products were presented and foreseen, and materials were blends with low weight fraction of NFC which prohibited to achieve high mechanical properties.